Electrochemical and in Situ Ellipsometric Investigation of the Permeability and Stability of Layered Polyelectrolyte Films

Abstract

Utilization of layered polyelectrolyte films as sensor or ion-separation materials will depend critically on their stability and ion permeability in aqueous solution. We report electrochemical and in situ ellipsometric studies on the permeability and stability of poly(allylamine hydrochloride)/poly(styrenesulfonate) (PAH/PSS) and PAH/poly(acrylic acid) (PAA) films. The permeability of these layered polyelectrolyte films to Fe(CN) 6 3 - and Ru(NH 3 ) 6 3+ depends on the solution pH, the number of bilayers in the film, whether supporting electrolyte is present during film deposition, and the nature of constituent polycations and polyanions. Cyclic voltammetry and impedance spectroscopy show that film permeability is similar in pH 3.2- and pH 6.3-buffered solutions but increases dramatically in alkaline solutions. In situ ellipsometry helps to explain these results. Upon immersion in pH 3.2- and pH 6.3-buffered solutions, the thickness of PAH/PSS films increases by 40%, but swelling is constant over time. At pH 10, these films initially swell by 40% but then continue to swell for several minutes before delaminating. The onset of increased swelling corresponds with dramatic increases in film permeability. Both peak current (cyclic voltammetry) and charge-transfer resistance (ac impedance) depend nonlinearly on the number of polyelectrolyte bilayers. The structure of the first two bilayers is more porous than that of later bilayers. Adding supporting electrolyte to deposition solutions results in thicker bilayers and changes film permeability. For PAH/PSS films, the use of supporting electrolyte during film formation results in a much less permeable film (comparing films of similar thickness). In PAH/PAA films, however, the use of supporting salt results in highly permeable films (even with thicknesses as high as 44 nm). Thus proper choice of constituent polyelectrolytes and deposition conditions permits control over the permeability of layered polyelectrolyte films.